AI oh n Warren 5 
not the case, as can be seen in a wax reconstruction of the parts, Fig. 8. 
This is a reconstruction of the brain of an embryo of 14.5 mm. The tops 
of the hemispheres, 7, have been removed to give a clear view of the 
paraphysis, P, which otherwise would be more or less covered in by them. 
The paraphysis appears as a straight tube in the median line and caudad 
to it is seen a broad partition, V, extending the whole width of the dien- 
cephalon. This is the velum, consisting of one layer only, which répre- 
sents the two originally distinct cephalic and caudal layers. The down 
growth of the parts in order to provide room for the development of the 
paraphysis has formed a deep angle in the roof of the fore brain. This 
Fic. 8. Wax model of brain of embryo of 14.5 mm. Harvard Embryo- 
logical Collection, Sagittal Series, * 120 times. 
angle is bounded caudad by the velum and cephalad or ventrad by the 
narrow roof of the telencephalon (paraphysal arch) immediately 
cephalad to the paraphysis. As the hemispheres develop, they grow at 
first in a dorsal direction and occupy the space left by the formation of 
this angle, so that the paraphysis is practically buried between the hemis- 
pheres in front and the velum behind, Fig. 10. The growth of the 
paraphysis must, therefore, be regarded as having an important effect on 
the development of the fore brain at this stage. 
Up to this stage the development of the velum has been in a ventral 
direction towards the floor of the fore brain, but now it begins to grow in 
quite a different direction. In Fig. 7 a distinct bulging of the velum is 
